1. Technical Field
The present invention relates to a method of designing a progressive power lens group as a set of progressive power lenses for covering an insufficiency of ability of accommodation owing to presbyopia, in which the grouped progressive power lenses are based on the same design concept.
2. Related Art
A progressive power lens has: a far-distance use section in its upper portion for seeing in a relatively long range; a near-distance use section in its lower portion for seeing in a relatively short range; and a progressive section located in the middle of the far-distance use section and the near-distance use section and having a gradually changed refractive power. In general, the distance between the far-distance use section and the near-distance use section, namely the length of the progressive section, is referred to as progressive zone length. In a progressive power lens, because the boundary of the far-distance use section and the near-distance use section is not located on the lens, it is difficult to determine the size of the progressive zone length from the lens. However, in the description hereof, a portion of transition from a region where the refractive power is substantially constant to a region where the refractive power changes between the far-distance use section and the progressive section on a major meridian, is defined as a far-distance use section lower end. And a transitional portion from the changing refractive power region to a near-distance use region where the refractive power region is substantially constant between the progressive section and the near-distance use section, is defined as a near-distance use section upper end. The distance between the far-distance use section lower end and the near-distance use section upper end shall be referred to as “progressive zone length.”
As shown in FIG. 2, a progressive power lens 10, which is delivered to a distributor, has a layout print used in fitting an eyeglass into an eyeglass frame, a fitting point F, a refractive power reference circle for measurements of a far-distance use refractive power and an addition, etc. printed thereon using an erasable ink. The center of the far-distance use refractive power reference circle 1 is referred to as far-distance use refractive power measurement point Dp, and the center of the addition's reference circle 2 is referred to as near-distance use refractive power measurement point Np. In general, the fitting point F is provided at a transition point between the far-distance use section and the progressive section, and the upper end of the addition's reference circle 2 is substantially coincident with the near-distance use section upper end, which somewhat differs from eyeglass manufacturer to manufacturer. On this account, the distance from the fitting point F to the upper end of the addition's reference circle 2 in a vertical direction can be simply regarded as a progressive zone length.
It is common that the region for the near-distance use section is provided in a peripheral portion located out of a center portion of a lens. In the peripheral portion of a lens, a very large prism refractive power arises under the influence of the far-distance use refractive power and addition of the lens. A group of progressive power lenses, in which the lenses are standardized and designed under a certain design concept so that base elements as a progressive power lens has, such as a far-distance use clear-vision region width and a near-distance use clear-vision region width, accomplish a common wearing purpose, is provided to consumers, for example, under the same trade name. The progressive zone length of each progressive power lens of a group of progressive power lenses having the same trade name is constant. In other words, the location of the near-distance use section is fixed regardless of the value of the far-distance use refractive power.
Incidentally, for example, as shown in JP-A-2003-329984, there has been the idea of changing the amount of biasing the near-distance use section toward the center depending on the prism effect of the near-distance use section previously. However, this is arranged so that left and right sight lines when a user sees with both eyes overlap properly. Also, prior to JP-A-2003-329984, inventions concerning the layout print which can cope with the change in location of the near-distance use section have been disclosed in e.g. JP-A-2001-51241, JP-A-2002-311396, JP-A-2003-131175, and JP-A-2003-131176. However, in any of them only the displacement corresponding to the near-distance use biasing toward the center is stated.
However, the prism amount at the near-distance use refractive power measurement point Np is changed depending on the far-distance use refractive power and the addition and as such, the location of an object point when the object is seen through the near-distance use refractive power measurement point Np is changed depending to the refractive power. For example, with a concave lens having a minus refractive power, the location of an object point differs between the sight line L1 heading for a rotation center O of an eyeball when an object is seen through the concave lens 11 shown by a solid line, having a relatively plus refractive power in near-distance use section refractive power and the sight line L2 heading for a rotation center O of an eyeball of the progressive power lens 12 shown by a broken line, having a relatively minus refractive power in near-distance use section refractive power, as shown in FIG. 3A. Therefore, in order to see a particular object with such concave lenses differing in refractive power, it is needed to change a wearing angle of the lens thereby to make the locations at which the object point is seen coincide, as shown in FIG. 3B. For example, to make the location of an object point of the progressive power lens 12 having a relatively minus refractive power in near-distance use section refractive power coincide with that of the concave lens 11 having a relatively plus refractive power in the near-distance use section refractive power, the progressive power lens 12 having a relatively minus refractive power in the near-distance use section refractive power has to be turned up somewhat to see an object point through the near-distance use refractive power measurement point Np.
This applies to a convex lens having a plus refractive power, too. As shown in FIG. 4A, the location of an object point differs between the sight line L3 shown by a solid line when an object is seen through the convex lens 21 shown by a solid line, having a relatively plus refractive power in the near-distance use section refractive power and the sight line L4 shown by a broken line through the progressive power lens 22 shown by a broken line, having a relatively minus refractive power in the near-distance use section refractive power. Therefore, in order to see a particular object with such convex lenses differing in refractive power, it is needed to change a wearing angle of the lens thereby to make the locations at which the object point is seen coincide, as shown in FIG. 4B. In either case, as for lenses for spectacles, it is required to turn the user's face up and down thereby to enable seeing an object through the near-distance use refractive power measurement point Np.
As described above, a progressive power lens has a portion to see in near-distance range located in a peripheral portion of the lens where the prism refractive power becomes larger, and a user has to change the tilting angle of his or her head depending on the refractive power of the lens to see. As a result, immediately after a lens is replaced with a new one of a group of progressive power lenses having the same trade name, the user would feel that the lens is bothersome and inconvenient until the user becomes used to the new lens.